how do I ground a 25V circuit?

Thread Starter

unsaint32

Joined Jan 13, 2004
14
I have 277/480 Y at work and I need to hook up an exit sign that uses 25V. So, I am planning to use a transformer, I assume 277/25V transformer.
I looked up the code book which said, I need to ground a system if it is less than 50V, and if it is fed by a supply system of more than 150V.
But, how do I system ground the secondary side, which is the exit sign? Run a grounding wire to concrete? Even if I did that, what do I connect it to? All the electrodes, the building metal structure, and water pipes are being used as electrodes for the building.

Am I wrong about the code?

Jon


What is the "objectionable current" at the code
 

Erin G.

Joined Mar 3, 2005
167
I have 277/480 Y at work and I need to hook up an exit sign that uses 25V. So, I am planning to use a transformer, I assume 277/25V transformer.
I looked up the code book which said, I need to ground a system if it is less than 50V, and if it is fed by a supply system of more than 150V.
But, how do I system ground the secondary side, which is the exit sign? Run a grounding wire to concrete? Even if I did that, what do I connect it to? All the electrodes, the building metal structure, and water pipes are being used as electrodes for the building.

Am I wrong about the code?

Jon


I assume that the 480 volt is the primary (high) side of your supply transformer, and that the 277 volt is the secondary (low) side. All of the exit signs that I've worked with use 110 volt (AC) input, and have their own, internal transformer to step down to 25 volts. If you're sure that you need 25 volts on the input, here's what to do,...

First, you'll have to order a transformer, because that's probably an odd duck voltage drop (277 / 25), and you'll have to make sure that you get the right voltamp (va) rating for the fixture you're installing. When you install the 277 / 25 transformer you'll have to ground the case of that x-former to the equipment ground of the supply side. This can probably be done by bonding the case of your new x-former to the conduit that's feeding it the 277 from the supply. You need to make sure that that conduit is bonded at the 480 / 277 x-former as well. (The 480 / 277 x-former should already have an equipment ground.)

The output of your 277 / 25 x-former will probably have two leads, x1 and x2, more if it's a multi voltage secondary. Regardless, you'll only be using two wires for your 25VAC. Simply bond one of them to the same conduit we talked about above. You now have an equipmment ground for your new tranformer, as well as an electrical ground for the 25VAC side of the x-former.
 

AENEUMANN

Joined Feb 15, 2007
2
The way that most LED night lights are powered from 115VAC is to use a capacitor.
For example, suppose the LED needs 10mA and the regulator circuit needs 10mA, or 20 Ma total.
Calculate the size of capacitor which when connected to the 115V line and grounded to AC neutral & Earth ground combined (for safety) will draw 20mA RMS.
The capacitor feeds a simple rectifier (could be a full wave bridge) to produce about 5V. If a single diode rectifier is used then the AC current must be doubled.
The 5V regulator can be a simple zener diode. Connect the LED to the +5V supply with a limiting resistor such that +5V - 1.3V (LED drop) will result in the desired 10mA.

This same method can be used to construct a very simple and potentially dangerous battery charger. Suppose you desire to charge up a typical auto battery at 5 Amps. Procure a 100uF motor-start capacitor rated for about 300V AC. connect this to a zip cord (AC cord) and then to a full-wave bridge. Out of the bridge comes a pulsating current of 5 Amps RMS. If the output is limited by say about 1 ohm and connected to the battery before the zip cord is plugged in then OK. However if the zip cord is plugged in first, then you have a 170V DC 5-Amp source that will electrocute just about anything. To make the thing safe, add to the output of the bridge rectifier na 1 ohm and then a power 20V Zener diode. This zener will limit the voltage to a safe level however still above what the battery needs.
 

kubeek

Joined Sep 20, 2005
5,719
This same method can be used to construct a very simple and potentially dangerous battery charger. Suppose you desire to charge up a typical auto battery at 5 Amps. Procure a 100uF motor-start capacitor rated for about 300V AC. connect this to a zip cord (AC cord) and then to a full-wave bridge. Out of the bridge comes a pulsating current of 5 Amps RMS. If the output is limited by say about 1 ohm and connected to the battery before the zip cord is plugged in then OK. However if the zip cord is plugged in first, then you have a 170V DC 5-Amp source that will electrocute just about anything. To make the thing safe, add to the output of the bridge rectifier na 1 ohm and then a power 20V Zener diode. This zener will limit the voltage to a safe level however still above what the battery needs.

I don´t think that this is a type of idea to be presented to many unexperienced people. If the zener fails, your thought-to-be safe charger becomes deadly again.
 

wireaddict

Joined Nov 1, 2006
133
Yes, you need to ground the step-down transformer because of the supply voltage level. More about that later. Your building power is known as a 480Y277 V, 3-phase system & 277 V is the voltage from any phase to your neutral or "Y" junction, which is grounded. Transformers with 277 V primaries aren't as common as those with 480 V or 240/480 V so you'll probably wind up with one of these. Ditto for 25V secondaries; 24 V is a common voltage & is near enough to 25 V to work fine. Also, make sure your tranny's big enough to supply all your lights if there's a chance you'll be powering more than one. Multiply the total current of of all the light fixtures by 1.25X25 [31.25] to get right size, in VA, then, if that size is non-standard, get the next larger one.

As for grounding, the source for your 277 or 480 V is probably a grounded enclosure or junction box. Erin G. is right, get an enclosed tranny or put it in a metal box. Conduit will work as a grounding coundutor for your tranny enclosure as will your steel building framework & mounting hardware. Also, run a short jumper wire from a tranny mounting screw to a secondary terminal or lead; if the terminals or leads are marked, X2 is the customary grounded lead. Make this wire white to indicate that it's grounded & the grounding jumper green.

I also recommend that you feed the transformer primary from a fused disconnect switch & it would be a good idea to fuse the ungrounded secondary lead to protect the tranny from shorts.
 
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